Adhesive, sealing and coating compositions and pretreatments with a primer on the basis of polyurethane prepolymers which are humidity curing are known and are broadly used for coating, connecting and sealing building and construction materials such as e.g. plastics, glass, ceramics, varnished sheet metals, metals, wood, concrete and other substratums.
Such compositions do advantageously comprise little or no solvents and they contain isocyanate groups comprising prepolymers which are prepared in a known manner by reacting bifunctional or polyfunctional polyols with an excess of diisocyanate or polyisocyanate, whereby the monomers content in the whole formulation is as low as possible, i.e. smaller than 1%, preferably smaller than 0.5%.
In the presence of e.g. humidity, the isocyanate groups of the monomers as well as the isocyanate groups at the ends of the polyurethane (PU) prepolymer chains react with water under formation of an instable carbamic acid group that spontaneously decomposes to amines and carbon dioxides. Said amino group then does fast react with a further isocyanate group under the formation of a urea group. Said cross-linkage reaction causes a molecule growth and leads to a hard or tough elastic composition suitable for adhesive sealing and coating purposes.
By the admixture of catalysts, the reaction of water with isocyanate groups can be accelerated. Known catalysts are titanates, organometal compounds such as e.g. tin or lead compounds that can also be combined with other catalysts, particularly tertiary amines. Generally the catalysts are used in amounts of up to 2% referred to the whole formulation.
If large amounts of catalyst are used to speed up the curing throughout the layer, on the one hand the stability of the PU system is affected, making an application after short storage time impossible due to an enhancement in the viscosity. On the other hand the temperature stability of the cured composition is reduced due to depolymerisation. Desired is a PU system that is still applicable after a storage time of more than 6 months.
U.S. Pat. No. 4,780,520 describes the use of dimorpholinodiethylether (DMDEE) as catalyst for the formulation of a storage stable fast curing PU system, whereby DMDEE is used in an amount of 0.2 to 1.75% referred to the whole composition.
GB patent application No. 2,231,879 shows that the use of 0.2 to 2% of tetramethyl substituted DMDEE also enables the formation of a storage stable PU-system, whereby the strength development at low temperature and low humidity, i.e. at 5.degree. C. and 50% relative humidity, is faster than with a DMDEE catalysed PU-system.
In U.S. Pat. No. 4,705,840 2,2'-dimorpholinylalkylethers are disclosed which at 24.degree. C. and 55% relative humidity show the same early strength as DMDEE, i.e. 7.5 minutes after application of an orthopaedic bandage, at half the concentration of DMDEE, i.e. with 5% by mole.
The use of the above mentioned catalysts for the formulation of fast curing PU systems with fast strength development has the disadvantage that the assembly time is reduced to below 8 to 10 minutes. The assembly time, also termed open time, is defined as the time between the application and the assembling for which a good adhesion is granted. The skinning time according to experience is shorter than or identical with the assembly time and is an efficient method for the approximate determination of the assembly time (see examples).